1. Field of the Invention
The present invention relates to component mounting substrates on which components such as BGA (Ball Grid Array) and LGA (Land Grid Array) packages are mounted by use of solder joints.
2. Description of the Related Art
Generally speaking, a QFP (Quad Flat Package) has terminals (lead pins) on its package sides, which terminals are to be soldered to a substrate, thereby establishing connection between the QFP and the substrate. In addition, such lead pins can absorb drop impact applied to the package and substrate as well as a stress caused by a thermal expansion difference between the substrate and the component.
With recent demand for downsized components and devices, integrated circuit packages (hereinafter also called “components”) such as BGA (Ball Grid Array) packages and LGA (Land Grid Array) packages having no such lead pins formed thereon have been increasingly used. Such BGA and LGA packages have solder joints, or so-called balls or lands, projecting from the undersurface of their packages, through which solder joints the packages are mounted and connected to substrates (printed board; component mounting substrate).
Although components are connected to substrates by use of solder joints (balls or lands), this connection is nevertheless not so different from direct connection, in which components are directly connected to substrate. This makes it difficult to absorb drop impact or the like applied to the components, and to absorb stress applied thereto by temperature change, such stress being caused by a difference in thermal expansion rate (thermal expansion coefficient) between the substrates and the components.
Accordingly, such substrates on which BGA or LGA packages are mounted are susceptible to drop impact and temperature change, so that damages such as solder cracks and exfoliation of electrodes are likely to happen at connections (soldered parts) between the substrates and the components. This disadvantage lowers the reliability of such component mounted substrates.
FIG. 7 and FIG. 8 are cross-sectional views showing a previous component mounting substrate and a previous component mounting structure. The substrate 1 of FIG. 7 and FIG. 8 is mounted with a component (BGA in this example) 4 having no terminals (lead pins) to be soldered, and on the component side of the substrate 1 there are formed a substrate resist 3 and substrate electrodes 2. The component 4 is provided with component electrodes 5 having solder joints (balls) 7 that are to be soldered to the substrates electrodes 2 of the substrate 1, and the solder joints 7 and the substrate electrodes 2 are joined together with solder, whereby connection between the substrate 1 and the component 4 is established. Here, the component 4 also has a component resist 6 formed on a side thereof which is joined with the substrate 1.
In order to increase the aforementioned reliability of the component mounting structure, resin 8 is filled into a gap between the substrate 1 and the component 4, as shown in FIG. 7, to reinforce the structure, or hardware 9 is arranged around the component 4, as shown in FIG. 8, to increase the apparent rigidity of the substrate 1, thereby minimizing warp of the substrate 1.
For the purpose of absorbing a stress applied to the substrate 1 and the component 4 by impact or by the difference in the thermal expansion rate between the substrate 1 and the component 4, the following techniques have been developed:
[Patent Application 1] Japanese Patent Application Publication No. 2000-277923;
[Patent Application 2] Japanese Patent Application Publication No. 2003-124389; and
[Patent Application 3] Japanese Patent Application Publication No. 2001-332646.
The patent application 1 discloses a BGA package having solder electrodes that are formed as projections made of resin; the patent application 2 discloses grooves that are formed around electrodes on the component side of the substrate; the patent application 3 discloses grooves that are formed around electrodes on the side of the substrate opposite the component side.
With recent demands for downsizing (in weight, thickness, and size) of so-called mobile products such as mobile phones and PDAs (Personal Digital Assistants), demand for downsizing of component mounting substrates for use in such downsized mobile products has grown.
However, in accordance with the previous arts of FIG. 7 and FIG. 8, since use of resin 8 and mounting hardware 9 increases both the weight and the height of the package, the aforementioned demand for weight-reduction and thickness-reduction will not be satisfied. Moreover, the techniques of FIG. 7 and FIG. 8 do not satisfy the aforementioned size-reduction demand, either, because the resin 8 often slightly leaks from the gap between the substrate 1 and the component 4 in the technique of FIG. 7, and because a space to install the mounting hardware is required in the technique of FIG. 8.
In order to improve the rigidity of the substrate itself, it is conceivable to use an inorganic substrate such as a ceramic board or to increase the thickness of the substrate. However, since both of these methods significantly increase the weight of the substrate, they are not suitable for use in downsized mobile products.
In the patent application 1, demands for weight reduction and thickness reduction in substrates are satisfied. However, the projection-like shape of the substrate electrodes of this application causes the following problem. When the substrate electrodes and the solder joints are joined together with solder, solder must be supplied to the projections with high accuracy. This not only makes the solder supply process difficult but also necessitates high accuracy in joining the component and the solder.
Solder print is normally used to supply solder for joining a component and a substrate, and such solder print is often displaced several μm to 150 μm. In addition, at the time of joining a component and a substrate, the component is pushed against the substrate, whereby solder put on the substrate electrodes slightly spreads over the component side of the substrate, and is resultantly deformed.
Accordingly, in the art of the patent application 1, even slight displacement of solder supply can cause the supplied solder to come off the projection parts. In such cases, the adhesive strength between the component and the substrate is deteriorated, thereby also affecting solder connection.
Similarly, in the art of the patent application 2, grooves formed around substrate electrodes deteriorate tolerances on solder supply positions and component connections, thereby making solder connection difficult. In addition, if solder is applied to positions shifted from the proper positions in the patent application 2, the solder will flow into the grooves formed around the substrate electrodes, thereby making it impossible to perform solder connection with reliability. This will necessitate performance of more accurate soldering processing. Moreover, the solder filled into the grooves will also affect absorption of the aforementioned applied stresses.
Further, since the art of the patent application 3 provides grooves only on the side of the substrate opposite the component side, it is impossible to absorb a stress applied to the joints (that is, solder part) between the substrate and the component, at which joint cracks and exfoliation of the electrodes are likely to occur.